A primary concern in epidemiologic studies of drug abuse is the development of measurement strategies that are reliable and valid. Not only is it important to develop effective measures of drug abuse outcomes, but also reliable, unbiased measures of related risk factors. Methodologic studies designed to address these concerns will increase confidence in estimates of prevalence and incidence of drug abuse as well as producing indicators of association that are unlikely to be biased. Little is currently known about the reliability of instruments that measure problems related to the use of illicit drugs, including dependence. In particular, there is a relative dearth of information pertaining to test-retest reliability. For this reason, we completed a test-retest study designed to assess the reliability of the substance abuse supplement to the 1991 National Health Interview Survey. This questionnaire consisted of 122 items that assess the use of various drugs in the lifetime and last year, then specifically addresses problems related to the use of marijuana and cocaine which allow generation of dependence diagnosis according to DSM-IIIR criteria. Two hundred seven individuals who entered the recruitment process at the ARC completed the questionnaire at baseline and two weeks later. A random sample of participants (n=97) completed an additional questionnaire two hours after responding to the first questionnaire. Two-week and two-hour test-retest reliabilities for marijuana and cocaine dependence diagnoses among drug users were generally high. The reliability coefficients (Kappa) for two-week retest of lifetime diagnosis for DSM-IIIR marijuana and cocaine dependence were 0.71 and 0.77, respectively. Two-week reliabilities for dependence diagnoses in the past year were 0.61 for marijuana and 0.79 for cocaine. Reliability coefficients were slightly higher for the two-hour test-retest: lifetime marijuana dependence diagnosis, Kappa = 0.83; lifetime cocaine dependence diagnosis, Kappa = 0.91; past year marijuana dependence diagnosis, Kappa = 0.82; and past year cocaine dependence diagnosis, Kappa = 0.91.